1. Field of the Invention
The present invention relates to a lightwave interference measurement apparatus used to measure an optical path length or distance.
2. Description of the Related Art
The conventional lightwave interference type measurement apparatus calculates a geometric distance between a reference optical path and a target optical path by dividing an optical length found by a wavelength of a light source and a phase found by a signal of an interference between reference light and target light, by a refractive index of air between a reference surface and a target surface. The refractive index of air can be obtained, for example, from an experimental formula described in Bonsch, Potulski, Metrologia, 35, 133-9, 1998, by measuring the temperature, the air pressure, the vapor pressure, and the carbon dioxide concentration, etc. using environmental detectors. When there are spatial and temporal distributions of these environmental factors due to insufficient air conditioning, a measurement value of the environmental detector does not always accord with an average value of a distributing environmental factor. As a result, the precision of the air's refractive index found by the above experimental formula lowers, and the calculation precision of the geometric distance consequently deteriorates. As one measure for correcting fluctuations of the refractive index caused by the distributing environmental factors, for example, Japanese Laid-Open Patent Application (“JP”)10-019508 calculates the geometric distance by automatically correcting the fluctuations of the air's refractive index on the target optical path and the reference optical path based on optical path lengths obtained with different wavelengths by utilizing two light sources having different wavelengths. JP 10-096601 automatically corrects the fluctuations of the air's refractive index on the target optical path and the reference optical path by utilizing optical path lengths obtained with three or more wavelengths, and calculates the geometric distance.
However, the method disclosed by JP 10-019508 augments a distance measurement error without the vapor pressure of 0 [Pa] or in dry air, and thus is useless to the general environments that have a vapor pressure distribution on the measurement optical path. On the other hand, the method disclosed by JP 10-096601 is applicable the wet environment but the measurement precision of the calculated geometric distance deteriorates by one digit order or higher when the wavelength of the light source and the detection precision of the phase are similar to those of JP 10-019508. In order to maintain the similar precision of the geometric distance, the light source needs a wavelength region of a CO2 laser, for example, and becomes impractically bigger.